To an increasing extent, high strength and super high strength steels are used for example in the automotive industry, as said steels can be provided with lower wall thicknesses resulting in weight advantages. In order to form this material, hot forming is particularly suitable, which however requires additional coating of the blank so that these do not scale in the hot forming tool. An AlSi coating is frequently used. This effectively prevents the scaling of the blank during hot forming, in which the blank is heated at least to austenitising temperature. What is problematic when welding blanks coated in this way is that the components of the coating, in particular aluminium, penetrate the weld region and in the case of hot forming with subsequent press hardening lead to a lack of strength in the welded joint. The regions necessary for the decoating of the blanks have a main extension direction, namely in the direction of the welded seam.
The width of the regions is preferably limited to a maximum of 5 mm as otherwise the absence of the coating in these regions would in turn have negative consequences.
There are now various options to remove a coating, in particular an AlSi coating but also any other coating in the region of the welded seam from the blank before welding. These can be divided into thermal, chemical and mechanical methods.
For example, it is known that the coating in the desired regions can be removed by etching with acid. However, this is very time-consuming as the remaining regions of the blank on which the coating is to remain have to be masked or covered.
In terms of mechanical methods, longitudinal planning or milling of the coating have been attempted to date. However, both methods are very time-consuming and cost-intensive as the regions of the blank provided for the welded seams have to be worn down separately. In other words, the regions have to be shaved or milled along their main extension direction.
Furthermore, tests have been carried out which process the affected positions using sandblasting. As a disadvantage, it has been determined that parts of the coating, in particular aluminium, are pressed into the substrate.
A further method is described in the prior art in which the coating is removed by means of high frequency electromagnetic fields (DE 10 2008 006 624 A1). This method has yet to become established.
The evaporation of the coatings in these regions using a pulsed laser continues to be used. From the German utility model DE 20 2007 018 832 U1, for example, it is known not to completely remove the coating but rather to leave an intermediate layer known as the inter-metallic intermediate layer, on the substrate, which protects the decoated region from corrosion at least for a time-limited period. This inter-metallic intermediate layer further contains components of the coating such as aluminium. The remaining parts of the coating, for example aluminium, can then penetrate the welded seam. Furthermore, removing the coating using a laser is relatively time-intensive as the entire region has to be decoated along its main extension direction.
A mechanical method for removing the coating in the region of welded seams is further known from German utility model DE 20 2007 018 832 U1 in which the coating is removed mechanically using brushes. Again the disadvantage is that the entire welded seam has to be worn down with the brushes in order to decoat said welded seams over the entire length. In addition to this, when using the brushes it is not possible to ensure that the coating has been removed completely. All of the previously mentioned methods to remove the coating are very time-consuming and therefore result in relatively high costs.
On this basis, the object of the present invention is to provide a method which is as simple as possible and a device which is as simple as possible with which the coating of a blank can be removed effectively and cost-efficiently in one operation if possible.